Field of the Invention
Embodiments of the present invention relate to an imaging apparatus and an imaging method and, more particularly, to a technology suitable for use in an imaging apparatus having a clamping function.
Description of the Related Art
Herein below, the prior art will be described. The conventional imaging apparatus includes a lens that causes an incident light to form an image, an image sensor that converts an optical image formed by the lens into an electrical signal, and a signal processing unit that obtains a predetermined image signal by effecting various signal processing, after analog-to-digital (AD) conversion of the electrical signal obtained from the image sensor. Furthermore, the conventional imaging apparatus includes a clamp processing unit for compensating a black level of a video signal after the AD conversion processing. The clamp processing unit implements clamp processing, by using the video signal in an optical-black region shielded from light, of the image sensor.
A dynamic range exists in the clamp processing unit. When a signal of equal to or higher than a certain signal level, or a signal of equal to or lower than the signal level is input into the clamp processing unit, the value will exceed a range within which clamp processing is enabled. As a result, the clamp processing unit may not appropriately perform the clamp processing. When the signal level is amplified by an amplifier circuit at a front stage of the clamp processing unit, because of, for example, high temperature of the image sensor is, and significant dark current shot noise, the video signal may exceed the dynamic range before input of the clamp processing unit. In this case, the imaging apparatus only needs to offset the video signal at the front stage of the clamp processing unit, and to perform processing for causing the video signal to fall within the dynamic range of the clamp processing unit. A technology relating to clamp processing of the imaging apparatus is discussed in, for example, Japanese Patent Application Laid-Open No. 2000-358170.
An imaging apparatus discussed in Japanese Patent Application Laid-Open No. 2000-358170, based on an average value of signal levels of an optical-black portion of an image sensor in an image signal, corrects the signal level of the image signal. Then, the imaging apparatus subtracts the video signal before the clamp processing, and implements processing for causing the video signal to fall within the dynamic range of the clamp processing unit.
However, in a case where the imaging apparatus, when determining an average value (correction amount), in this manner, calculates an unique correction amount in a mix of four colors (R, Gr, Gb, and B) of sensor output video signal, and subtracts the same correction amount from the entire video signal, the following problems occur. The clamp processing unit is composed of a recursive type filter circuit, and has a certain time constant. Recursive processing requires a time according to the time constant to clamp the entire screen by a video signal of an optical-black region serving as a reference signal.
When the unique correction amount as described above is subtracted from the video signal, difference of video signals for each color after subtraction may becomes large. In a case where the video signal has entered into the clamp processing unit, it requires a time according to the time constant of the recursive type filter, to perform clamping. For this reason, the clamp processing unit may not instantaneously withdraw the video signal to a pedestal level, and there is a problem that a flicker occurs in a portion of the screen failed to withdraw.
Such a phenomenon occurs particularly on the condition under which level difference for each of four colors is likely to occur, such as a case where a gain of the amplifier at the front stage of the clamp processing unit is great, due to a high temperature and a low illuminance. When the time constant of the recursive type filter of the clamp processing is made short, for the purpose of countermeasures against the above-described problem, conversely, adverse effect of erroneous clamping occurs resulting from, for example, dark current and shot noise.
It is, therefore, desirable to enable appropriate clamp processing, even when the video signal which exceeds the dynamic range of the clamp processing has been input, without the need to make short the time constant of the recursive filter of the clamp processing unit.